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The delineation of high flood hazard zones within a flood plain is usually independent of the hydraulic parameters that constitute a life threatening situation. In order to defme human instability in high hazard areas, a study was conducted to identif' when an adult human could not stand or maneuver in a simulated flood flow. An analysis was performed on a rigid body monolith resulting in a toppling hazard envelope curve (velocity vs. depth). A 120 lb monolith was then constructed and tested to relate the actual flow velocity and depth at toppling to theory. A series of human subjects (90-20 1 ibs) were placed in a recirculating flume and tested to determine the velocity and depth of flow that caused their instability.The test results determined that the product number, which is the product of the velocity and depth at toppling of the monolith, closely compared to the theoretical envelope curve. The monolith results represent the lower limit of human stability. Also, the product number appeared to be a predictor of human instability in flood flow. A relationship was developed to estimate the product number at which a human subject becomes unstable as a function of the height and weight of the subject. (KEY TERMS: flood hazard; human stability; product number.) 1Paper No. 89010 of the Water Resources Bulletin. Discussions are open until April 1, 1990. 2Respectively, Professor,

Portable bedload traps ͑0.3 by 0.2 m opening͒ were developed for sampling coarse bedload transport in mountain gravel-bed rivers during wadable high flows. The 0.9 m long trailing net can capture about 20 kg of gravel and cobbles. Traps are positioned on ground plates anchored in the streambed to minimize disturbance of the streambed during sampling. This design permits sampling times of up to 1 h, overcoming short-term temporal variability issues. Bedload traps were tested in two streams and appear to collect representative samples of gravel bedload transport. Bedload rating and flow competence curves are well-defined and steeper than those obtained by a Helley-Smith sampler. Rating curves from both samplers differ most at low flow but approach each other near bankfull flow. Critical flow determined from bedload traps is similar using the largest grain and the small transport rate method, suggesting suitability of bedload trap data for incipient motion studies.

[1] This study empirically quantified the effect of sampling time on measured transport rates and fitted rating curves based on results obtained from bed load traps deployed for 2, 10, and 60 min in a coarse-bedded stream. As expected for a skewed distribution of transport rates, 2 min deployment underpredicted transport rates obtained from 10 and 60 min deployment by factors of 2 and 3 at moderate flows (50-70% Q bkf ). At near-bankfull flow the underprediction by 2 min versus 10 min sampling increased to a factor of 5, while transport rates collected during 60 min deployments were reduced because of overfilled bags. At flows near incipient gravel motion, 2 min sampling overpredicted transport rates obtained by 10 and 60 min deployment by factors of 2.7 and 3.4. The overprediction is attributed to computational effects arising mainly from the lowest measurable transport rate for each sampling time. Rating curves fitted to transport rates from 2 min sampling were significantly less steep than those for longer deployment times. However, sampling time explains only a small degree of the large difference between rating curves from bed load traps and a Helley-Smith sampler (2 min sampling).Citation: Bunte, K., and S. R. Abt (2005), Effect of sampling time on measured gravel bed load transport rates in a coarse-bedded stream, Water Resour. Res., 41, W11405,

Improved sampling techniques are needed to increase the accuracy of pebble‐count particle‐size distributions used for stream studies in gravel‐bed streams. However, pebble counts are prone to operator errors introduced through subjective particle selection, serial correlation, and inaccurate particle‐size measurements. Errors in particle‐size measurements can be minimized by using a gravel template. Operator influence on particle selection can be minimized by using a sampling frame, 60 by 60 cm, in which sampling points are identified by the cross points of thin elastic bands. Serial correlation can be minimized by adjusting the spacing between the cross points and setting it equal to the dominant large particle size (=D95). In a field test in a cobble‐bed stream, the sampling frame developed in this study produced slightly coarser size distributions, particularly in the cobble range, than the traditional heel‐to‐toe walk that selects particles with a blind touch at the tip of the boot. The sampling frame produced more similar sampling results between two operators than heel‐to‐toe walks. The difference between the two sampling methods is attributed to an unbiased selection of fine and coarse particles when using the sampling frame.